It is known in the art of medical radiography to employ intensifying screens to reduce the X-ray dosage to the patient. Intensifying screens absorb the X-ray radiation and emit electromagnetic radiations which can be better absorbed by silver halide emulsion layers. Another approach to reduce the X-ray dosage to the patient is to coat two silver halide emulsion layers on the opposite sides of a support to form a duplitized radiographic element.
Accordingly, it is a common practice in medical radiography to use a radiographic assembly consisting of a duplitized radiographic element interposed between a pair of front and back screens.
One of the problems of medical radiography relates to the different X-ray absorption of the various pads of the body. For example, in chest radiography the heart area has an absorption ten times higher than the lung area. A similar effect occurs in the radiography of the stomach, where a contrast medium is used in order to enhance the image depictivity (the body part having no contrast medium being totally black), and of hands and legs, where bones have an X-ray absorption higher than that of soft tissues such as flesh and cartilage.
In these cases a radiographic element showing a low contrast is required for area of high X-ray absorption and a radiographic element showing a high contrast is required for area of low X-ray absorption. The resulting film is a compromise in an attempt to have sufficient optical density and sharpness for these different areas of the body. In the art of chest radiography, X ray images providing visually discernible features in both the heart and lung image areas are attempted by using extended latitude radiographic elements. Extended latitude radiographic elements typically employ polydispersed silver halide emulsions to provide lower average contrasts and a wider range of exposures separating minimum and maximum density exposures. Said extended latitude radiographic elements, however, do not provide the desired sensitometric curve necessary to obtain visually useful imaging details in both head and lung areas. Various methods have been suggested to solve this problem. One approach relates to the use of double coated radiographic elements having a different emulsion layer coated on each side of the support. An example of this solution can be found in French patent 1,103,973, wherein the use of screens having a light emission ratio of from 1:1 to 1.5:1 (back screen:front screen) in combination with a radiographic element having coated thereon a high contrast back emulsion and a low contrast front emulsion is suggested. A combination of screens having a light emission ratio higher than 1.5:1 and radiographic elements having emulsion layers with the same gradation is also suggested. Other patents disclose the use of double coated radiographic elements having emulsion layers with different contrast or sensitivity. For example, DE 1,017,464 discloses a double coated radiographic element having coated thereon a first emulsion with high sensitivity and low contrast and a second emulsion with low sensitivity and high contrast, FR 885,707 discloses a double coated radiographic element having coated thereon a first high speed emulsion and a second high contrast emulsion, and FR 875,269 discloses a radiographic assembly comprising several radiographic films or papers, each having a different sensitivity and/or contrast relative to the others, in order to obtain separate and different images of the same object with a single exposure. Nothing in the above described patents suggests the use of the specific combination of the present invention to obtain a symmetric double-coated radiographic element showing a variable contrast according to the specific pair of intensifying screens. An approach similar to that of the above described French and German patents is disclosed in U.S. Pat. No. 4,994,355, claiming a double coated radiographic element having emulsion layers with different contrast, in U.S. Pat. No. 4,997,750, claiming a double coated radiographic element having emulsion layers with different sensitivity and in U.S. Pat. No. 5,021,327 claiming a radiographic assembly wherein the back screen and emulsion layer have a photicity at least twice that of the front screen and emulsion layer, the photicity being defined as the integrated product of screen emission and emulsion sensitivity. All these proposed solution require the use of an asymmetrical radiographic film which requires a specific orientation relative to the screens for a proper use.
The following are additional documents illustrating the state of the art.
FR 787,017 discloses a radiographic element comprising silver halide emulsion layers of different color sensitivity to be combined with intensifying screens emitting radiation to which the silver halides are sensitive. The purpose of this patent is to obtain a total use of radiation.
EP 88,820 discloses a radiographic fluorescent screen comprising a first blue emitting phosphor layer and a second green emitting phosphor layer to be combined with a silver halide element having spectral sensitivity in the blue-green region ("ortho-type" elements).
JP 60-175000 discloses a combination of a double coated silver halide element and a screen pair wherein the fluorescent layers of the two screens have different wavelength region emissions and each screen comprises an organic dye to absorb the light emitted by the opposite screen.
EP 350,883 discloses a technique for crossover reduction in which silver halide emulsion layers having different color sensitivities are provided on the opposite sides of a transparent support, and X-ray fluorescent intensifying screens having emission spectra corresponding to the respective color sensitivities are used.
Research Disclosure, December 1973, Vol. 116, Item 11620 discloses a radiographic element which shows different contrast when observed with or without a green filter, respectively.
Finally, EP 126,664 describes a radiographic material having a characteristic curve whose gamma between optical densities of 0.50 and 1.50 is 2.7 to 3.3 and gamma between optical densities of 2.00 and 3.00 is 1.5 to 2.5, said material having a wide exposure latitude to make possible the production of images having high diagnosis ability.